macOS: expand tilde in file paths before opening

`URL(filePath:)` treats `~` as a literal directory name, so
cmd-clicking a path like `~/Documents/file.txt` would fail to
open because the resulting file URL doesn't point to a real file.

Use `NSString.expandingTildeInPath` to resolve `~` to the user's
home directory before constructing the file URL.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

macos/Sources/Ghostty/Ghostty.App.swift

@@ -695,7 +695,10 @@ extension Ghostty {
             if let candidate = URL(string: action.url), candidate.scheme != nil {
                 url = candidate
             } else {
-                url = URL(filePath: action.url)
+                // Expand ~ to the user's home directory so that file paths
+                // like ~/Documents/file.txt resolve correctly.
+                let expandedPath = NSString(string: action.url).standardizingPath
+                url = URL(filePath: expandedPath)
             }
             
             switch action.kind {